This invention relates to an improved rotary kiln. More particularly it relates to an improved refractory lined rotary kiln wherein either or both ends of the cylindrical steel outer shell of the kiln are exposed to a substantially greater heat flux than the steel shell remote from such end or ends is exposed to.
The refractory lining provides most of the steel shell with good insulation against the intense heat within the kiln. Moreover, over most of the kiln length, the steel shell is exposed to the atmosphere which cools the steel shell. It is extremely difficult, however, to adequately shield the ends of the steel shell against such heat, and since the ends of rotary kilns are normally enclosed by hoods, the ends of the steel shell do not receive the benefit of atmospheric cooling. The resulting temperature differential between the end of the steel shell and the portion of the steel shell remote from this end results in more severe thermal expansion of the end of the steel shell in both the longitudinal and circumferential (with respect to the longitudinal axis of the rotary kiln) directions, placing severe mechanical stresses upon the refractory lining which usually is fixedly attached to the steel shell, often resulting in premature cracking and failure of the refractory.
While this can be a problem no matter what the operating temperature profile within the kiln may be, it becomes especially severe when the temperature profile within the kiln is such that either or both ends thereof are operated at temperatures significantly higher than in adjacent zones within the kiln.
For example, the problem can be particularly severe with direct-fired rotary kilns where one or more burner nozzles project into a burner hood at one end of the kiln. As a result of the intense heat from such burner, the cylindrical steel kiln shell at the burner end, even though lined with refractory is normally heated to a substantially higher temperature than the portions of the steel shell which are more remote from the burner end. Thus, this end of the kiln is often subjected to extremely great thermal expansion which causes greater expansion of the end of the steel shell in both the longitudinal and circumferential (with respect to the longitudinal axis of the kiln) directions and places severe mechanical stresses upon the refractrory lining which normally is fixedly attached to the steel shell, often resulting in premature cracking and failure of the refractory.
This problem becomes particularly acute in direct-fired rotary kilns for pyrolysis of carbonaceous materials wherein air, oxygen or another oxygen-containing gas is admitted to the kiln at the burner end for the in situ combusion of part or all of the pyrolysis gases. In such kilns the burner may be used only for start-up with the in situ combustion of the pyrolysis gas providing all of the thermal energy necessary to effect pyrolysis, or the burner may be in continuous or intermittent operation as a source of supplemental thermal energy after steady-state pyrolysis is attained, depending upon process requirements. In any event, extremely high temperatures, e.g., above 1900.degree.-2200.degree. F are frequently present in the burner end of the kiln because of the combusting pyrolysis gases.
Similar problems are often encountered at the other end of the rotary kiln as well. For example, extremely hot off-gases from the kiln can subject this end of the rotary kiln to a great heat flux as well.